Cool temperatures over the last couple of weeks have brought a slow down to growing degree day (GDD) accumulation for corn. Up until early August, GDDs for corn were accumulating at approximately the average rate. Since then, temperatures in the major corn growing region in South Dakota have been three to five degrees or more below average.
As a result, most areas in Eastern South Dakota are about 100 GDD below average for this time of year. The Corn GDD Tool at the High Plains Regional Climate Center (previously at the Useful 2 Usable website) can provide up-to-date GDD for every county.
Hard Frost Risk
Given the fact that many areas are below average for GDDs for corn, there is some risk of hard frost (minimum temperature of 28° F) occurring before black layer maturity. At the current time, black layer is estimated to occur before the average frost date in most areas. The East Central counties around the Brookings area have the highest risk of hard frost occurring before black layer, with about 20 to 35 percent probability (Figure 1). Most other counties have one to 15 percent risk of hard frost occurring earlier than maturity.
Figure 1. 2017 Corn GDDs for Brookings county. If corn was planted on April 10 with 100 day maturity variety, there is a 34 percent likelihood of a hard frost occurring before black layer maturity. This is indicated by the vertical black solid line that estimates black layer date, with a range between the vertical black dotted lines. Blue vertical lines show historical frost date, with the median date shown with the tallest blue line.
Source: High Plains Regional Climate Center